1. Field of the Invention
The present invention relates to a pre-drive circuit for a brushless DC single-phase motor appropriately for use as a fan motor to outwardly discharge heat generated within the casing of electronic equipment and, more particularly, to a pre-drive circuit for supplying a control signal to a switching element in a drive circuit of the motor.
2. Description of the Related Art
In office automation apparatuses such as personal computers and photocopying apparatuses, a number of electronic components is mounted in a limited space available within the casing thereof, and heat generated from the electronic components builds up in the casing, possibly damaging the electronic components.
Ventilation holes are opened in the side wall and top wall of the casing of the electronic equipment, and a fan motor is installed in the ventilation hole to discharge heat from within the casing.
Brushless DC single-phase motors are typically employed as a fan motor. A conventional pre-drive circuit for such a brushless DC single-phase motor is discussed below with reference to FIG. 3.
Referring to FIG. 3, a pre-drive circuit is a circuit portion other than a coil (motor coil) L1 for a brushless DC single-phase motor and a drive circuit 31 thereof. There are shown a DC power supply +B for motor driving, and a DC power supply +Vcc for driving the circuit.
As shown, the drive circuit 31 includes four switching elements N-channel MOS type power FETs (Field-Effect Transistors) PF1-PF4, a diode D31, and a capacitor C31.
The coil L1 is mounted on a motor stator (not shown), and is driven by a current from four power FETs PF1 through PF4 in the drive circuit 31 in a predetermined ON/OFF timing. The coil L1 thus generates a dynamic magnetic field (a rotating magnetic field).
The rotor (not shown) of the motor is provided with a permanent magnet, and is rotated as the permanent magnet rotates in step with the rotation of the rotating magnetic field.
The pre-drive circuit includes dedicated integrated circuits IC1 and IC2, resistors R31 through R35, capacitors C32 through C35, and diodes D32 through D35. Each of the power FETs PF1-PF4 contains a parasitic diode, as shown.
In the discussion that follows, the dedicated integrated circuits IC1 and IC2 are simply referred to as dedicated IC1 and IC2, respectively, and power FETs PF1 through PF4 are simply referred to as PF1 through PF4, respectively.
The dedicated IC1 receives a rotary position signal x of the motor (of the rotor, namely, the permanent magnet) detected by an unshown Hall effect device, a high-intensity signal y for shutdown, and a duty factor setting signal z for controlling the motor rotational speed. The dedicated IC1 receives a stepped up voltage VB1 discussed later, and turns on and off PF1 and PF3 in the timing determined by the signals x, y, and z.
The dedicated IC2 also receives signals x, y, and z. The dedicated IC2 receives a stepped up voltage VB2 discussed later, and turns on and off PF2 and PF4 in the timing determined by the signals x, y, and z.
Since PF3 and PF4, from among PF1 through PF4, are grounded at the sources thereof, PF3 and PF4 are turned on as long as the gates thereof (control input terminals) are slightly higher in voltage than the ground. PF1 and PF2 are arranged on the side of the power supply +B with respect to the coil L1. In the normal operating conditions under which a drive voltage of the coil L1 is approximately equal to a power supply voltage (VB), the gates of PF1 and PF2 need to be supplied with a voltage equal to or higher than-the power supply voltage (namely, a sum of the power supply voltage and a gate-source voltage to turn on PF1 and PF2).
To receive a voltage other than the power supply voltage, a power supply circuit becomes complicated in design and large in size, and costly. For this reason, any voltage is preferably prepared within the power supply circuit itself.
A voltage stepup circuit such as a charge pump circuit is thus added. A circuit of the diode 32, the capacitor 34, and the resistor R31, and a circuit of the diode D33, the capacitor C35, and the resistor R31 respectively form such a charge pump circuit.
The stepped up voltage VB1 from the node of the diode 32 and the capacitor C34 is fed to the dedicated IC1 as a stepped up voltage VB for turning on PF1. The stepped up voltage VB2 from the node of the diode D33 and the capacitor C35 is fed to the dedicated IC2 as a stepped up voltage VB for turning on PF2.
The dedicated IC1 feeds, at the gate of PF1, a high-voltage pulse signal HO responsive to the voltage VB at a predetermined ON/OFF timing, and the dedicated IC2 feeds, at the gate of PF2, a high-voltage pulse signal HO responsive to the voltage VB at a predetermined ON/OFF timing. The gates of PF3 and PF4 receive low-voltage pulse signals LO responsive to the power supply voltage (Vcc) from the dedicated IC1 and IC2 at predetermined ON/OFF timings.
The ON/OFF timings are set within the dedicated IC1 and IC2 in response to the signals x, y, and z. Signals from the dedicated IC1 and IC2 respectively turn on and off PF1 through PF4 at a predetermined duty factor at the predetermined timing, thereby feeding a current to the coil L.
The motor (the rotor) is thus rotated in a predetermined direction in accordance with the signals x, y, and z. The motor has a fan, and is mounted at a ventilation hole of a casing of electronic equipment. The motor is then rotated as a fan motor to outwardly discharge heat from within the casing of the electronic equipment.
Such a conventional circuit is costly because of its dedicated IC1 and IC2.
IC1 and IC2 are both bulky. With the bulky IC1 and IC2 and other individual electronic components, the circuit requires a large mounting space. When the circuit is applied to a small motor, it is difficult to mount the large two IC1 and IC2 and other electronic components on an associated small circuit board.
The present invention has been developed in view of this problem, and it is an object of the present invention to provide a pre-drive circuit for a brushless DC single-phase motor, which is low-cost, requires no large mounting space, and is easy to mount on a small printed wiring board.
To achieve the above object, a pre-drive circuit of the present invention for a brushless DC single-phase motor controls a motor rotational speed by changing a duty factor of an ON/OFF control voltage to switching elements. The pre-drive circuit drives a brushless DC single-phase motor drive circuit including a pair of series connections of switching elements being connected between a power supply and ground, each series connection formed of two switching elements, and a motor coil connected between the two nodes, each node of the two switching elements in each series connection, wherein the motor coil is controlled with a current fed therethrough at any timing in any direction in an ON/OFF manner by turning on and off the switching elements, and a control voltage exceeding the voltage of the power supply is needed to turn on two power-supply-side switching elements. The pre-drive circuit includes a voltage stepup circuit for stepping up the power supply voltage to a predetermined voltage, a logic circuit for generating and then outputting pulse signals for controlling the switching elements, based on a motor rotary position signal and a duty factor setting signal for controlling the motor rotational speed, a pair of operational amplifiers which are respectively connected to output terminals of the logic circuit for the pulse signals for controlling the two power supply side switching elements, are supplied with the stepped up voltage from the voltage stepup circuit as a power source, amplify the pulse signal for controlling the two power-supply-side switching elements to a predetermined voltage level above the power supply voltage, and respectively feed the amplified pulse signals to control input terminals of the two power-supply-side switching elements, and a pair of ground-side switching element driving signal input circuits which are respectively connected to pulse signal output terminals on the logic circuit for two ground-side switching elements, and feed, to control input terminals of the two ground-side switching elements, the pulse signals from the pulse signal output terminals directly or via resistors.
Preferably, the voltage stepup circuit includes a charge pump circuit, and the pre-drive circuit includes a resistor and a capacitor for removing noise connected between the output terminal of the charge pump circuit and ground.
Preferably, the pre-drive circuit includes a voltage limiting element connected between the output terminal of the charge pump circuit and ground.